Centrifugal pump

ABSTRACT

An impeller for a centrifugal pump which includes a front shroud and a rear should, the shoulds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades. The impeller has an outer diameter D 2  and an inlet diameter D 1 . Each passageway has an inlet portion with a passage outlet B 2 , and an intermediates portion between the inlet and outlet portions. In the inlet portion the front shroud is curve away from the rear shroud so that the passageway outlet width B 2  is less than the passageway inlet width B 1 . A method of increasing the Best Efficiency Point Flow rate comprises providing such an impeller and also providing a new front liner or throat bush which has an inner wall which is complementary to the wall of the inlet portion of the impeller while retaining the same main liner or casing section.

This invention relates to centrifugal pumps and more particularly, butnot exclusively to slurry pumps.

BACKGROUND OF THE INVENTION

The invention is particularly applicable to centrifugal pumps having aninternal liner although reference to this particular application is notto be taken as a limitation to the scope of the invention. It will bereadily apparent to those persons skilled in the art that the inventionis also applicable to pumps which do not have an internal lining.

FIG. 1 is a schematic sectional side elevation of part of a typicalcentrifugal slurry pump currently in use. The pump generally indicatedat 10 comprises an elastomeric liner 11 which is mounted within a rigidhousing (not shown). The liner 11 includes a main liner 12 and a frontliner 13 (often referred to as a throat bush). The main liner may beformed of two parts. Such is well known in the art and it is notproposed to discuss it in detail here.

The pump 10 further includes an impeller 15 comprising a front shroud 16and rear shroud 17. A series of passageways 18 are formed between theshrouds these passages being separated from one another by blades orvanes 19. The pump 10 has an inlet 20 and each passage has a passagewayinlet 21 and a passageway outlet 22. The pump inlet 20 is shown ashaving a diameter D₁, the passageway inlet 21 is shown as having a widthB₁ and the passageway outlet is shown as having a width B₂. The outerdiameter of the impeller is shown as D₂.

All centrifugal pumps have a flowrate at which their efficiency is at amaximum. This is called the Best Efficiency Point (BEP) flowrate.

FIG. 2 is a graph for a typical centrifugal pump plotting the head (orpressure) of the pump against flow rate. The BEP flowrate is that whenthe graph reaches its highest point.

At lower or higher flows, the efficiency is less than at the BEP point.The BEP flowrate is determined by the pumps geometry. The most practicaland cost effective method of producing pumps is to design pumps with afixed geometry to suit a particular duty. Normally the pumps BEPflowrate will be made to coincide as close as possible to the requiredor duty flowrate in order to achieve the most economical operation.

Once a pump's geometry is fixed, then the BEP flowrate can only bechanged to a small degree. The design and manufacture of variablegeometry centrifugal slurry pumps is not economical. Changing theinternal liner shape of the configuration of the impeller is possible inorder to make small changes to the BEP flowrate. However, such changesare expensive as patterns and molds require alteration to change thegeometry. This particularly applies to the pump liners.

In some instances, the required or duty flowrate specified by a customeris higher than the BEP flowrate for the available fixed geometry pumps.In this case the efficiency will be lower than optimum and would resultin higher running costs. This situation might arise if the duty flowrateis higher than the largest pump available, or the duty flowrate fellbetween two fixed pump models. In both cases it is logical to increasethe BEP flowrate of the smaller pump if the increase required is in theorder of up to 35% higher.

The BEP flowrate is determined amongst other parameters by the width ofthe pump liners and the impeller. To increase the BEP flowrate, theimpeller needs to be made wider. As it is not practical or economical tochange the main pump liners, the outlet width of the impeller cannot beincreased.

Typically it will only be the flowrate that needs to be increased andthe head (pressure) and speed of the pump would remain approximately thesame. Increasing only the pump flowrate, increases a pumps specificspeed. This is a non-dimensional number incorporating the pump flowrate,head and speed and is universally applied to characterize a pump'sdesign. The specific speed and hence the pump head can be improved bychanging the design of the impeller.

Typically in currently known centrifugal pumps the widths of thepassageway inlet and outlet are approximately the same. Furthermore theinclination angle β as shown in FIG. 1 is in the range from 0 to 15°.

The inclination angle is defined as the angle between a line joining themid points of the passageway inlet and outlet widths to a line at rightangles through the passageway outlet width.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved pumpwhich has the same impeller diameter (D₂) and passageway outlet width(B₂) which has an increased flowrate at its BEP relative to a currentlyknown radial flow pump.

It is another object of the present invention to provide an improvedimpeller for use in a pump according to the present invention.

According to one aspect of the present invention there is provided animpeller for a centrifugal pump which includes a front shroud and a rearshroud the shrouds being spaced apart so as to form a plurality ofpassageways therebetween which are separated by a plurality of impellerblades the impeller having an outer diameter D₂ and an inlet diameterD₁, each passageway having an inlet portion with a passageway inlethaving a width B₁ an inlet portion having a passageway outlet B₂ and anintermediate portion between the inlet and outlet portions;characterized in that in the inlet portion the front shroud is curvedaway from the rear shroud so that the passageway outlet width B₂ is lessthan the passageway inlet width B₁.

Preferably, the passageway angle (B) (as herein defined, is in the rangefrom 10° to 35°. In one preferred arrangement the passageway angle isabout 20°.

Preferably the ratio of D₂ /D₁ is from 1.5 to 3 and the ratio B₁ /B₂ isfrom 1.1 to 1.6.

According to another aspect of the present invention there is provided apump having a casing with or without main liner and front liner and animpeller as described above.

By the above arrangement the width of the impeller at its passagewayinlet can be increased without affecting the main liner or casing.Modification is only required of the front liner or throat bush. Thesemodifications are much cheaper than having to modify the main liner orcasing. By modifying the impeller as described above has the effect ofincreasing the BEP flowrate and increasing the pump's specific flowrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, already described, is a schematic sectional side elevation of aknown centrifugal slurry pump;

FIG. 2, already disclosed, is a graph for a known pump plotting pumphead against flow rate;

FIG. 3, is a view similar to FIG. 1 of a pump according to theinvention, and

FIGS. 4 and 5 show plots of the head or lift of the FIG. 3 pump againstflow rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An example embodiment of pump according to the present invention isshown in FIG. 3 where like reference numerals have been used to describelike parts as shown in FIG. 1.

As shown in FIG. 3, the pump 10 has an impeller 15 which includespassageways 18 which include an inlet portion 21, an outlet portion 22and an intermediate portion 23. The walls of the passageways form acontinuous smooth curve from the outlet portion to the inlet portion. Ascan be seen the width B₁ of the inlet is greater than the width B₂ ofthe outlet and the angle β is greater than that of the currently knownpump shown in FIG. 1.

Design practicalities of slurry pumps, generally dictate that the widthof the impeller at the inlet and the outlet is approximately the same(i.e. B₁ =B₂) in FIG. 1. The inclination angle β for a normal radialdesign of slurry pumps is 0 to 15°. The angle β is defined as shown inFIGS. 1 and 3 as the angle between a line joining the mid points of theinlet and outlet widths to a vertical line through the outlet width midpoint. The inlet width (B₁) is sometimes increased in normal practice toimprove the pumps cavitation performance. Ratios of inlet to outletwidth (B₁ /B₂) could typically vary from 1.0 to 1.15.

If the inlet is "stretched", then the inlet to outlet width ratio (B₁/B₂) can be increased and the angle β of the passageway can beincreased. There is an optimum ratio at which the increased BEP flowrateis achieved beyond which there is a diminishing increase in BEPflowrate. The casing design can also affect the final result. A largewidth ratio would be B₁ /B₂ =1.1 to 1.6. The angle β would vary between10 and 35° with an optimum angel around 20° for a D₂ /D₁ ratio of 2 to2.5. As the D₂ /D₁ ratio becomes larger, the practicality of stretchingthe inlet would become less and the lower the angle that β that could beachieved.

The impeller vane design must also be in line with a mixed flow typepump and to match the new higher flowrate. The front liner and casinghalf of the pump would also be changed as necessary to match te newangle of the impeller.

While the method is economical for lined slurry pumps, the sameprinciples could be applied to unlined pumps.

EXAMPLE

A comparative test was done between a conventional pump having animpeller of the type shown in FIG. 1 with a pump having an impeller ofthe type shown in FIG. 3. Relevant parameters of each pump are set outbelow

    ______________________________________                                                Conventional Pump                                                                        Modified Pump                                              ______________________________________                                        D.sub.2   1425         1435                                                   D.sub.1   625          690                                                    B.sub.1   325          470                                                    B.sub.2   325          408                                                    ______________________________________                                    

FIGS. 4 and 5 show plots of head (lift) against flow rate.

It can be seen that at the best efficiency (BEP) for each pump themodified pump at a head of 25 meters has significantly increased flowrate compared to that of the conventional pump at the same head.

Finally, it is to be understood that various alterations, modificationsand/or additions may be incorporated into the various constructions andarrangements of parts without departing from the spirit or ambit of theinvention.

What is claimed is:
 1. A method of increasing the flow rate at BEP for a radial flow pump of selected parameters said pump including a main casing and a front casing or throat bush having an inlet diameter D₁ and an impeller having an outer impeller diameter D₂ and an inlet diameter substantially the same as the inlet diameter of the front casing or throat bush, the method including the steps of:(a) replacing the impeller with a modified impeller, said modified impeller including a front shroud and a rear shroud the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades the impeller having an outer diameter D₂ and an inlet diameter D₁, each passageway having an inlet portion with a passageway inlet having a width B₁ an outlet portion having a passageway outlet B₂ and an intermediate portion between the inlet and outlet portions; in the inlet portion the front shroud being curved away from the rear shroud at a passageway angle β so that the passageway outlet width B₂ is less than the passageway inlet width B₁ ; and (b) replacing the front casing or throat bush with a modified front casing or throat bush having an inner wall which is complementary to the wall of the front shroud of the modified impeller while retaining the same main casing.
 2. A method according to claim 1 wherein the passageway angle β is in the range from 10° to 35°.
 3. A method according to claim 2 wherein the passageway angle is about 20°.
 4. A method according to any one of claims 1 to 3 wherein the ratio of D₂ /D₁ is from 1.5 to 3.0.
 5. A method according to any one of claims 1 to 3 wherein the ratio B₁ /B₂ is from 1.1 to 1.6. 